Esters of mono-alcohols derived from plant or animal oils have numerous applications. The main application to be pointed out is that of substitute fuels for gas oil in which the usual oils can be used as starting materials, such as rapeseed oil and sunflower seed oil, and also oils containing a high proportion of saturated fatty acids linked to glycerine, such as palm oil or tallow, to produce high purity esters. However, in that application, the presence of even trace amounts of saturated mono-glycerides of fatty acids is particularly deleterious, as mono-glycerides tend to crystallise over time and thus block filters.
In this regard, the invention proposes a process which can produce esters with a much reduced mono-glyceride content, and which also has the advantage of enabling a very pure glycerine to be produced which can be directly used in industrial or food applications without distillation.
When using esters as substitute gas oil fuels, it is also desirable to be able to use used cooking oil as the starting materials which initially contain both heavy products (dimers, trimers) and free fatty acids. The process of the invention can produce very pure esters from such starting materials.
Other applications have also been researched, such as those concerning the production of ester bases which, by ester hydrogenolysis, can produce alcohols or dimers or amides by dimerisation or amidation.
Methyl esters can be produced by conventional homogeneous catalysis techniques using soluble catalysts, such as sodium hydroxide or sodium methylate, by reacting a neutral oil and an alcohol such as methanol (JAOCS 61, 343-348, 1984).
Two main processes have been industrialised.
The first process consists of working under conditions which converts oil to methyl esters in very high yields. It is not necessary to evaporate the ester to eliminate the secondary products formed, especially when dealing with an unsaturated ester, in which the majority of fatty acids linked to the alcohol are unsaturated, i.e., oleic or linoleic acid type. In this case, traces of mono-glycerides which are present remain soluble in the ester.
A second process consists of evaporating off or distilling the ester at the end of the first transesterification step. To prevent the mono-glycerides present in the ester from being entrained, they are transformed into di- and/or tri-glycerides by reacting the mixture of esters obtained, free of alcohol or glycerine, at a temperature of 200.degree. C. or more in the presence of a soluble basic catalyst. Under these conditions, soaps form with the alkaline catalyst, such that only a portion of the distillation or evaporation residue can be recycled, since partial elimination is necessary in order no to accumulate alkaline derivatives and heavy derivatives (esters of sterols present in proportions which vary depending on the nature of the oil).
It should be added that, in order to purify the glycerine, the two processes necessitate multiple steps, as the glycerine is polluted by alkaline salts or alcoholates, and the glycerine purification apparatus is almost as expensive as that for producing the ester.
Processes using heterogeneous catalysis employing a fixed bed have been described in the literature, in different patent documents (European patent EP-B-0 198 243, British patent GB-A-0 795 573, EP-B-0 198 243 and French patent FR-A-2 752 242).
Such processes have the advantage of producing esters and glycerine which are free of catalyst and are thus easy to purify. However, it has been shown that to obtain complete conversion of an oil to the ester, two steps generally have to be carried out which include a plurality of operations; thus, after the first transesterification step, the excess alcohol has to be evaporated off, the glycerine formed has to be eliminated, then alcohol has to be re-injected with the partially converted ester, the mixture is heated again and, after reacting, the alcohol is again evaporated off and the glycerine is settled out. To obtain complete conversion of the oil to the ester in one step, a very large excess of alcohol would be required along with a very long residence time (very low HSV). If all of the operations described above are not to be carried out, it appears to be preferable to distil or evaporate the ester after the first transesterification step. The crude ester can be distilled but this necessitates heavy investment in a plate column with the known risk of overheating the bottom of the column and a large energy consumption mainly due to an unavoidable reflux.
If evaporation or flash evaporation using a falling-film evaporator is used, there is a risk of entraining traces of mono-glycerides in the ester with a boiling point which is close to that of the ester, in particular when in the presence of esters with C.sub.18 acids and mono-glycerides of C.sub.16 acids which is the case when starting from certain oils which are rich in palmitic acid.
It is difficult to introduce alkaline products which can transform the mono-glycerides into heavier products into the ester, as is the case with homogeneous catalysis, since the alkaline products will pollute the heterogeneous catalyst and even the products produced such as glycerine.